1. Field of the Invention
The present invention is related to a power converter, and particularly to a control circuit and method for the power converter.
2. Brief Description of the Related Art
Due to the semiconductor technology being developed progressively, the digital products such as the computer and the peripherals thereof are capable of being upgraded continuously. The fast change of the manufacturing process for the semiconductor results in a variety of demands for the power source of the integrated circuit (IC) employed in the computer and the peripherals thereof. Hence, the pulse width modulation voltage regulators composed of various power converters such as the boost converter and the buck converter to meet the need of different integrated circuits become one of the most important factors to determine if versatile digital products are capable of being presented.
The output voltage of a power converter should be maintained at a steady state instead of rising or dropping corresponding to change of the load during the power converter working normally. However, when the type of the output of the power converter during the load being drawn is instantaneous step change, most part of the current output of the power converter is provided with the output capacitor and it results in the voltage rising or dropping rapidly.
For solving this problem, literatures “Z. Zhao, A. Prodic, Continuous-time digital controller for high-frequency DC-DC converters, IEEE TRANSACTIONS ON POWER ELECTRONICS, March 2008” and “E. Meyer, Z. Zhang, Y.-F. Liu, An optimal control method for buck converters using a practical capacitor charge balance technique, IEEE TRANSACTIONS ON POWER ELECTRONICS, JULY 2008” have proposed the nonlinear control, and the authors entitle it as time-optimal control (TOC) method. When the instantaneous step change occurs during the load being drawn, the power switch is fully ON/OFF a period of time, and it is expected that the inductance current and the output voltage are capable of meeting the steady state of the output specification at end of the time-optimal control.
In addition, the literature “A. Costabeber, L. Corradini, P. Mattavelli, S. Saggini, Time optimal, parameters-insensitive digital controller for DC-DC buck converter, PESC, 2008” proposes two TOC methods suitable for buck converter, time-based TOC method and voltage-based TOC method. The fundamental theories of the two control methods are based on the wave shapes of voltage and current shown in FIG. 1 and stated hereinafter.
Steps of the time-based TOC method are detecting if change of the output voltage Vo of the power converter exceeds a default value ΔVth, i.e., t0 in FIG. 1, and starting the control if the change exceeds the default value ΔVth, detecting the lowest point Vvalley of the output voltage Vo of the power converter and setting a time interval T1 between t0 and time corresponding to the lowest point Vvalley, figuring out Q0 and Q1 of reducing voltages of the output capacitor and Q2 and Q3 of increasing voltages of the output capacitor according to the principle of electric charge equilibrium and setting Q0+Q1=Q2+Q3 such that it can be expected that Vo restores to the original value thereof so as to calculate the time interval T1+T2 of the power switch being fully ON and the time interval T3 of the power switch being fully OFF. The related equation is expressed in the following:
  {                                                                                                                                                            C                        ·                        Δ                                            ⁢                                                                                          ⁢                                              V                        th                                                              +                                                                  ∫                        0                                                  T                          1                                                                    ⁢                                                                        ∫                          0                          t                                                ⁢                                                                                                                                            V                                in                                                            -                                                              V                                ref                                                                                      L                                                    ⁢                                                                                                          ⁢                                                      ⅆ                            τ                                                    ⁢                                                                                                          ⁢                                                      ⅆ                            t                                                                                                                                =                                                                                    ∫                        0                                                  T                          2                                                                    ⁢                                                                        ∫                          0                          t                                                ⁢                                                                                                                                            V                                in                                                            -                                                              V                                ref                                                                                      L                                                    ⁢                                                                                                          ⁢                                                      ⅆ                            τ                                                    ⁢                                                                                                          ⁢                                                      ⅆ                            t                                                                                                                +                                                                                                                                            ∫                    0                                          T                      3                                                        ⁢                                                            ∫                      0                      t                                        ⁢                                                                                            V                          ref                                                L                                            ⁢                                                                                          ⁢                                              ⅆ                        τ                                            ⁢                                                                                          ⁢                                              ⅆ                        t                                                                                                                                                                                                T                2                                            T                3                                      =                                                            V                  ref                                                                      V                    in                                    -                                      V                    ref                                                              =                              D                                  1                  -                  D                                                                          ⇒                  ⁢          {                                                                  T                2                            =                                                D                                ·                                                                            T                      1                      2                                        +                                                                  2                        ⁢                                                  LC                          ·                          Δ                                                ⁢                                                                                                  ⁢                                                  V                          th                                                                                                                      V                          in                                                -                                                  V                          ref                                                                                                                                                                                                            T                3                            =                                                                    1                    -                    D                                    D                                ·                                  T                  2                                                                        wherein Vref represents the reference voltage, Vin represents an input voltage of the buck converter, L represents the inductance, C represents the output capacitance, and D represents the duty cycle of the buck converter at steady state.
Steps of the voltage-based TOC method are detecting if change of the output voltage Vo of the power converter exceeds a default value ΔVth, i.e., t0 in FIG. 1, and starting the control if the change exceeds the default value ΔVth, detecting the lowest point Vvalley of the output voltage Vo of the power converter and setting a time interval T1 between t0 and time corresponding to the lowest point Vvalley, figuring out a state transfer point Vsw with the lowest point Vvalley, measuring and setting a time interval T2 between Vvalley and Vsw for calculating the time interval T1+T2 of the power switch being fully ON and the time interval T3 of the power switch being fully OFF. The related equation is expressed in the following:
         {                                                      V              sw                        =                                          V                ref                            -                                                (                                      1                    -                    D                                    )                                ·                                  (                                                            V                      ref                                        -                                          V                      valley                                                        )                                                                                                                    T              3                        =                                                            1                  -                  D                                D                            ·                              T                2                                                        wherein Vref represents the reference voltage of the buck converter, and D represents the duty cycle of the buck converter at steady state.
It is ideally supposed in the preceding methods that no parasitic resistance in the output capacitor of the buck converter. In case of the parasitic resistance being taken into account, the preceding literatures propose a correction to extend time duration Rc·C for the power switch being ON, but it has been mentioned that the correction is not applied to a larger parasitic resistance. Besides, when the inductance L of the buck converter is getting smaller and the output capacitor C and the parasitic resistance are getting larger, T1 detected with the time-based TOC method is 0, and the voltage-based TOC method is unable to figure out the voltage transfer point correctly due to the output voltage containing a voltage drop of the parasitic resistance. Hence, it is not possible for the output voltage of the power converter to restore to the steady state value at end of the TOC, and it results in the system triggering the TOC repeatedly and the circuit being incapable of working normally.